Wireless signal transmission is significant in a series of applications. By way of example, a large number of functions in motor vehicles are increasingly initiated or controlled by means of remote controls. Usually, a radio link is used in license-free frequency bands for the transmission from and to the motor vehicle. For vehicle entry and also for starting the engine, for example, these are what are known as remote keyless entry systems (RKE systems for short), as are used in radio central locking systems, for example. Local area radio networks such as WLAN networks (for example based on the IEEE802.11 standard) and GPS systems also make use of wireless signal transmission and, in the course of this, also frequently of transmission methods using band spreading. There are various band spreading transmission methods in existence, but currently two methods are most common.
Firstly, these are DSSS methods (DSSS: Direct Sequence Spread Spectrum), in which the symbol energy is distributed over a large bandwidth. To this end, the useful data stream to be transmitted is multiplied by the spread code, the chip rate of which is higher than the data rate of the useful data stream. The code sequence comprises chips—the information to be transmitted comprises bits. By way of example, pseudo random bit stream (PRBS) codes or pseudo static codes (PN codes) are used. The spreading means that a greater bandwidth is required for transmission. The longer the spread code, the more bandwidth is needed. At the same time, however, the energy density in the spectrum is reduced, which means that other signals are disturbed less. The useful data stream can be reconstructed at the receiver again only by using the correct chip sequence.
Secondly, the wireless data transmission also involves the use of CDMA methods (CDMA: Code Division Multiple Access). CDMA methods are code division multiplex methods for the simultaneous transmission of a multiplicity of useful data streams which all use the same frequency resource. The jointly simultaneously used frequency range has a much greater bandwidth than each individual useful data stream would require separately without spectrum spreading. In order to implement this relatively great bandwidth, band spreading methods are again used.
For the implementation of wireless radio links, over distances of 2 km, for example, there is the problem, particularly in the USA, that high transmission and radio powers (up to 1 W or 30 dBm) are admissible only for large frequency bandwidths that are used (e.g. >500 kHz).
In order to achieve use of the prescribed frequency bandwidth, the data are transmitted with the same data rate but with an increased chip rate in comparison with a narrowband transmission. The link budget of the transmission link is therefore not impaired in comparison with a narrowband transmission. By contrast, an increased data rate would result in lower receiver sensitivities and hence shorter system ranges. In order to be permitted to send at high transmission powers, e.g. in a frequency range from 902 MHz to 928 MHz, it is therefore necessary to use a band spreading modulation method such as DSSS or CDMA in order to distribute the transmission power over large frequency bandwidths (e.g. >500 kHz). A further advantage of band spreading transmission methods is that the useful data can be transmitted in a manner that is secure from eavesdropping.
In this case, a narrowband data signal is thus transmitted after having been subjected to band spreading and is despread again at the receiver. The basis for the receiver-end despreading is what is known as signal acquisition, which also includes the ascertainment of parameters that are required for the signal despreading (for example code phase, frequency errors, starting parameters for tracking mechanisms). Following successful signal acquisition, the signal can be despread and the bit string that has been sent can be detected.
In order to be able to detect the bit string/information to be transmitted at the receiver end, for example, it is necessary for the spread spectrum signal to be correlated to the same spread code, in particular. As a basis for the correct detection of the bit information, i.e. so that despreading of the wanted signal WS can be achieved, it is necessary for the phase difference between the spread code signal at the transmitter end and the spread code signal at the receiver end to be small, at best zero, during the correlation and hence during the despreading. Therefore, the time offset that can be assumed by the receiver (initial code phase) should correspond to a small code phase difference during the despreading. The receiver does not know the time offset that is to be assumed a priori, and said time offset is the main purpose of the signal acquisition.
During reception, it is necessary to take account of the inevitable presence of primarily narrowband disturbance signals, which influence particularly the radio channel that is used for the wireless transmission. These disturbance signals can hamper the ascertainment of the initial code phase at the receiver end, for example.
A particularly important receiver parameter is the signal-to-jammer ratio (SJR), i.e. the ratio of the powers of the wanted signal WS and a disturbance signal JS. The aim of any acquisition method is to keep down the signal-to-jammer ratio (SJR) at which successful acquisition is still possible. On the one hand, every band spreading modulation method has inherent disturbance signal rejection (spreading gain), which on its own already reduces the SJR, and on the other hand additional disturbance signal rejection which reduces the SJR further is desirable.
Methods are known which have the aim of applying narrowband disturbance signals to already known disturbance signals by using digital “adaptive rejection filters” (notch filters), for example. Since the actual dispreading needs to be preceded by the performance of disturbance signal recognition, the spreading gain that is inherent in this transmission method is not used in this case.